Overall objective is delineation of mechanisms responsible for the quantitative regulation of gene function in the hope that such knowledge may lead to methods for therapeutic redifferentiation. We suppose that gene function and "gene switching" reflect features of gene and intergene nucleotide sequences. We have developed two model systems which separately and in combination will allow the analysis of: (1) functional relationship between gene and intergene, (2) role of reiterated DNA in gene regulation, (3) mechanism of "gene switching", and (4) the construction of a detailed molecular map of individual chromosomes. The first system uses mRNA for individual globin chains derived from immunologically purified reticulocyte polysomes. Such mRNA can be obtained in large amount from a variety of species. Given near mg quantities of mRNA, globin genes and their neighboring nucleotide sequences can be obtained through hybridization of whole genome DNA with specific mRNA. By hybridizing DNA fragments of increasing length successively with, for example, beta-mRNA and gamma-mRNA, it will be possible to obtain DNA tracts which contain both genes as well as the "intergene" sequences lying between them. The second system involves the isolation of DNA from individual chromosomes in settings where a test of function is possible. In situations where two cell types are distinguished by a single chromosome -- such as male (XY) and female (XX) cells -- or mouse x human sometic hybrid cell lines containing a single chromosome or chromosome fragment -- such as the X or 17 -- it is possible to obtain DNA specific for the distinctive chromosome by competitive reassociation. This makes possible a detailed structural analysis of individual chromosome heretofore unattainable.